Objection 1. It would seem that omission is not a special sin. For every sin is either original or actual. Now omission is not original sin, for it is not contracted through origin nor is it actual sin, for it may be altogether without act, as stated above (I-II, 71, 5) when we were treating of sins in general. Therefore omission is not a special sin.
Objection 2. Further, every sin is voluntary. Now omission sometimes is not voluntary but necessary, as when a woman is violated after taking a vow of virginity, or when one lose that which one is under an obligation to restore, or when a priest is bound to say Mass, and is prevented from doing so. Therefore omission is not always a sin.
Objection 3. Further, it is possible to fix the time when any special sin begins. But this is not possible in the case of omission, since one is not altered by not doing a thing, no matter when the omission occurs, and yet the omission is not always sinful. Therefore omission is not a special sin.
Objection 4. Further, every special sin is opposed to a special virtue. But it is not possible to assign any special virtue to which omission is opposed, both because the good of any virtue can be omitted, and because justice to which it would seem more particularly opposed, always requires an act, even in declining from evil, as stated above (1, ad 2), while omission may be altogether without act. Therefore omission is not a special sin.
On the contrary, It is written (James 4:17): "To him... who knoweth to do good and doth it not, to him it is sin."
I answer that, omission signifies the non-fulfilment of a good, not indeed of any good, but of a good that is due. Now good under the aspect of due belongs properly to justice; to legal justice, if the thing due depends on Divine or human law; to special justice, if the due is something in relation to one's neighbor. Wherefore, in the same way as justice is a special virtue, as stated above (58, 6,7), omission is a special sin distinct from the sins which are opposed to the other virtues; and just as doing good, which is the opposite of omitting it, is a special part of justice, distinct from avoiding evil, to which transgression is opposed, so too is omission distinct from transgression.
Reply to Objection 2. Omission is not original but actual sin, not as though it had some act essential to it, but for as much as the negation of an act is reduced to the genus of act, and in this sense non-action is a kind of action, as stated above (I-II, 71, 6, ad 1).
Reply to Objection 2. Omission, as stated above, is only of such good as is due and to which one is bound. Now no man is bound to the impossible: wherefore no man sins by omission, if he does not do what he cannot. Accordingly she who is violated after vowing virginity, is guilty of an omission, not through not having virginity, but through not repenting of her past sin, or through not doing what she can to fulfil her vow by observing continence. Again a priest is not bound to say Mass, except he have a suitable opportunity, and if this be lacking, there is no omission. And in like manner, a person is bound to restitution, supposing he has the wherewithal; if he has not and cannot have it, he is not guilty of an omission, provided he does what he can. The same applies to other similar cases.
Reply to Objection 3. Just as the sin of transgression is opposed to negative precepts which regard the avoidance of evil, so the sin of omission is opposed to affirmative precepts, which regard the doing of good. Now affirmative precepts bind not for always, but for a fixed time, and at that time the sin of omission begins. But it may happen that then one is unable to do what one ought, and if this inability is without any fault on his part, he does not omit his duty, as stated above (ad 2; I-II, 71, 5). On the other hand if this inability is due to some previous fault of his (for instance, if a man gets drunk at night, and cannot get up for matins, as he ought to), some say that the sin of omission begins when he engages in an action that is illicit and incompatible with the act to which he is bound. But this does not seem to be true, for supposing one were to rouse him by violence and that he went to matins, he would not omit to go, so that, evidently, the previous drunkenness was not an omission, but the cause of an omission. Consequently, we must say that the omission begins to be imputed to him as a sin, when the time comes for the action; and yet this is on account of a preceding cause by reason of which the subsequent omission becomes voluntary.
Reply to Objection 4. Omission is directly opposed to justice, as stated above; because it is a non-fulfilment of a good of virtue, but only under the aspect of due, which pertains to justice. Now more is required for an act to be virtuous and meritorious than for it to be sinful and demeritorious, because "good results from an entire cause, whereas evil arises from each single defect" [Dionysius, De Div. Nom. iv]. Wherefore the merit of justice requires an act, whereas an omission does not.
Source: Summa Theologica (New Advent)